Method of producing improved calcium sulphite



Patented May 15,

2,375,786 mnrnon or raonuoma mnovnn CALCIUM SULP BITE Gerald Haywood, Weaternport, Md., and Wright M. Welton, Piedmont, W. Va., assig'nors to West Virginia Pulp and Paper Company, New York,

, N. E, a corporation of Delaware No Drawing. Application August 2, 1940,

1 Serial No. 350,156

18 Claims.

This invention relates to an improved method of producing calcium sulphite in a bright, finely divided state, satisfactory for use in the manufacture of paper and as a filler or pigment in inks,

- paints, protective coatings and the like. It relates V also to-the improved product resulting from the This product -is particularly denovel process. sirable as an ingredient of the coating composition applied to the surface of a web of paperin the production of coated papers. It is also highly satisfactory as afiller forpaper, in lieu of the resularly known fillers, such as china clay, calcium carbonate, and other types or calcium sulphite, although it may be more expensive to produce than these'other fillers. Asan ingredient of a coating mixture, the element of expense is not so important and the added expense, if any, of the. improved product is offset by other savings made pglssible through its use and by the improved re- 5 ts.

It has heretoforebeen suggested that calcium sulphite produced by a reaction between concentrated milk of lime and gaseous sulphur dioxide, brought about by contacting the milk of lime ln a finely divided state with an atmosphere relatively richin sulphur dioxide, may be used as a filler for paper or as an ingredient ofa coating composition.

proved process may be effectively bonded to the fibers of a sheet of paper through the use or a smaller amount of adhesive or size, such as casein, than prior known types of calcium sulphlte. Furthermore, coated papers produced through the use of the improved calcium sulphlte are superior in many respects to coated papers heretofore produced. i r

In accordance with the present invention, calcium sulphite is first produced in a finely divided state by any of the previously known methods for producing the product in a form suitable for use as a filler in paper. Preferably, however, it is produced, as previously explained, by the reaction of a relatively concentrated milk of lime suspension in the form of a fine spray with sulphur dioxide in a gaseous state present in an atmosphererich in sulphur dioxide. The method and apparatus preferably employed for this purpose are fully set forth inv the Haywood Patents No. 1,984,188 and No. 2,191,467 and in the co-pending Haywood application Ser. No, 756,631 filed Decembegii, 1934, now U. S. Patent 2,210,268. Any of the variations in the process and apparatus ,set forth as in said patents and application may be 101 Calcium sulphite so produced forms an excellent filler for paper, being brighter and capable -of producing greater opacity than commonly used fillers, such as china clay and chalk. However, it

is not entirely free from objection when it is to be used as an ingredient of a coating composition.

I The amount of casein or similar adhesive material required to bring about an appropriate bond between the coating and the fibers ofthe paper increases the cost of the coating composition unduly. This-is probably due to the porous character of the material and may also be due to the presence of impurities, particularly free lime, in the calcuimsulphite produced by the method set forth above. It has been an important obiectof the present inventionto produce calcium sulphite in such a state that it is highly satisfactory for use'in large percentage in a coating composition, It has been found that in the employment otthe'improyedprocess, calcium 'sulphite, may be produced in a more finely divided state'and of greater purity than previously known types of calcium sulphite. Its apparent density is also greater, e. g., 1 cc. of the-improved materialina dry state weighs in the neighborhood of 1.4 whereas the porous product of the prior methods under the same conditions has a weight only or about .5 ms per cc. It has been found that the product oi the 1mlowed. However,. the preferred procedure involves the reaction between a milk of lime suspension at a concentration of about grams per liter and an atmosphere consisting of about 17% sulphur dioxide and the balance-the products of combustion of sulphur in air in a sulphur burner.

A reaction chamber or absorber is used, the milk of lime being introduced into the absorber to an extent-suflicient only to occupy a small percentaseof the volume of the absorber while the atmosphere rich in sulphur dioxide occupies the major portion of the volume of the absorber.

Suitable agitating means are employed to whip.

the milk of hole into a fine spray and bring about the desired reaction between this spray and the sulphur dioxide. Preferably the milk of lime is continuously circulated, being withdrawn from the absorber at one point and returned at another. As explained in the prior patents and application, the concentration of, the milk, of lime may be varied, although it is preferably maintained between ya and 1 /2 pounds of lime per Ballon of water. In lieu of a milk of lime. suspension, other lime-bearing suspensions, such as a suspension of finely divided calcium carbonate,

may be employed. The reacting atmosphere may. contain between '7 and 20% sulphur dioxide and while it is preferably derived'by the burning of sulphur in the manner indicated. it may, if dell s d. e. derived imm otherlcuroqnch agar.

industrial plants,

Now, according to the prior methods disclosed in the patents and application mentioned, the

reaction is carried on until the lime has beeirsubstantially completely converted into calcium sulphite. The reaction may be carried to the point at which the suspension is slightly acid, in which case it is substantially neutralized by the addition of an alkali or the reaction may be arrested while the suspension is still slightly on the alkaline side and it may then be neutralized by a small amount of acid. The resulting product is extremely fine and highly satisfactory for use as a filler in paper but, as previously set forth, it is not entirely satisfactory as an important ingredient of a coating composition. According to the present invention, this material is greatly improved for use as a filler and more especially for use as an ingredient of a coating composition by the subsequent treatment now to be explained.

After the reaction between the milk of lime and the sulphur dioxide in the atmosphere derived from the sulphur burner is substantially complete, the suspension may either be passed to another chamber for further treatment or the further treatment may be carried out in the same absorber. This further treatment consists in reacting the suspension with gaseous sulphur dioxide, preferably of a concentration of between 50 and 100%. Such a by the distillation of liquid sulphur dioxide or may be the relief gases from a sulphite digester. In lieu of sulphur dioxide an acid may be employed which will release sulphur dioxide-by reaction with the calcium sulphite. Excellent results have been obtained by first removing the major portion of the suspending liquid from the original calcium sulphite precipitate, either or by filter pressing or by allowing the calcium sulphite to settle and then decanting the supernatant liquid, and forming a cake of i the calcium sulphite in a plastic state. This cake should be of sucha consistency that it maybe worked or puddled in a suitable vessel or chamber in the presence of the previously atmosphere of 'high S02 concentration. 'Itwill be found that the material under these conditions will at first become thicker but upon continuedworking it will become thinner again and will finally reach a state in which it may be pumped. Apparentlsythe structure ofthe material undergoes some change, in the course of the puddlin'g process, by which water is released or made available for increasing the fluidity of the material. The thinning of the material may also be due in part to the conversion of a substantial part of the original solids into calcium bisulphite. While the amount of time required for the extra sulphitation may vary with the results deproduced in the reaction. I In general, it has been residual calcium sulphite.

, although it may vary between 2.5 and 5.5. To

gas may be derived, for example,

consequently the more calcium bisulphite that is formed, the finer will be the particles of the The yield of the fine calcium sulphite, of course, diminishes with the increase in the excess of the sulphur dioxide employed. If as much as2 /2 mols excess of sulphur dioxide is employed, the yield of calcium sulphite will be about 60% of that resulting from the normal reaction between $02 and milk of lime. On the other hand, if only 4; mol excess of sulphur dioxide is employed, the yield of the residual calcium sulphite will be about 90% of the theoretical normal.

In any case it is desirable to carry out the treatment with excess S02 at a high concentration of the suspension. As explained, the water should be only sufficient to enable the material to be readily puddled. Preferably the concentration will be between 400 and 500 g./l; or even higher, although concentrations of CaSOa down to about 200 g./l. may be employed with fair results. In view of the high concentration, the

pl-I of the material will be relatively low, preferably in the neighborhood of 3 or between '3 and 4,

increase the solubility of the SOain the suspending water, the material is preferably kept as cool as possible during the extra sulphitation. For this purpose special cooling arrangements are preferably employed around the chamber in which the material is being puddled so that the temperature will be in the neighborhood of 50,

to 60 F. Satisfactory results may, however, be obtained at somewhat say, 100 F. v

After the extra sulphitation has been carried to the desired extent; the remaining calcium sulphite is removed from th suspending liquor in any suitable way, as by filtration. The sepmentioned arated, suspending liquid, containing a substantial amount of calcium bisulphite, may be utilized in a variety of diiierent ways. If desired, it may be reacted with milk of lime by pouring the bisulphite into a body of milk of lime and stirring the mixture thoroughly. The resulting precipitated calcium sulphite will befound satisfactory for'use as a filler in paper.

The calcium sulphite removed from the original suspending liquid after the over-gassing or extra sulphitation should be washed and neutralized by the addition of an appropriate quantitiy of an alkali, such as caustic soda, calcium carbonate or lime, This product, in addition to its sired and the amount of additional sulphur dif preferablywcarried on through the use of an excess of sulphur dioxide to-the extentof between A:

a I i and 2 mols based upon the calcium sulphite extreme-fineness, it being far bulkier, by a net bulk test, than the productsresulting from any previously known processes, has other improved characteristics as well. The extra sulphitation appear to. remove all traces of iron, the iron apparently going into the bisulphite solution. This leaves the, residual calcium sulphite in an extremely white and bright state, superior to prior types of calcium sulphite. Another important advantage of the over-gassed product is the ability to size it readily with casein or similar adhesive materials employed in the coating of paper. This will be made apparent by the tests which will be later explained.

In lieu of subjecting the original calcium sulphite produced, for example, in accordance with the methods set forth in the above-mentioned patents and application, to overgassing or extra sulphitation, it'has been found that somewhat higher temperatures up to,

asvsneo thecalcium sulphite pigment materials used.,

sheets '25 x 38 inches) was applied to each. The sheets so produced were subjected to a brightness,

results desired. Apparently, the more acid that is used, the finer will be the remaining calcium sulphite. Ordinarily, it will be iourid desirable to convert about 20% of the original calcium sulphite into the salt of the acidused, such as the chloride or sulphate or phosphate. However,

. good results may be obtained byconverting only ot the calcium sulphite into such salts and better results maybe obtained by converting between 30 and 40%, although this adds considerably to the expense. The same considerations of concentration, temperature and pH, as explained in connection with the over-gassing treat ment, should be? observed in the acid treatment.

The residual calcium sulphite removed from the acidis of a very bright character and is extremely fine. As in the case of the overgassed material, its bulk will be round considerably higher than the original calcium sulphit prior to the acid treatment. Moreover, the acid-treated material will be found capable of being readily sized with casein so as to makeit highly satisfactory for use as an ingredient of a coating composition. Another advantage of the overgassed or the acidtreated material is the elimination of the major portion of the free lime which is invariably present in ordinary calcium sulp'hite as heretofore produced. Even the overgassed or acid-treated material contains about 2% lime but this is not sufflcient to cause any serious difllculty in the materials indicated, in each instance utilizing casein to, the extent of 15% obthe dry weight of The same procedure was followed ineach instance in the production or thehand-coated sheets and a coating of about 15 pounds (for a ream oil 500 test on a General Electric brightness tester and to a pick test by the use of Dennisons graded waxes. The results of these tests are indicated inthe following table:

Ham Conan Su ars 15% Sizing (1) (2) Untreated mso. 301 so.

Brightness (G. E.)...-. 85.4 am. We (Dennison's).... i l coating. ifistock- #iistock "stock.

It will he noted that in the brightness tests the sheet .iormed with the untreated material was somewhat poorer than those formed with. the

treated materials and in the pick test the sheet mixture of .the calcium -sulphite with casein.

When an excessive amount of lime is present, however, it appears to react with the casein in such a wayasto greatly increase the viscosity 0! the mixture and make it diflicult to brush or cause to flow.

As a specific example of the conduct of the improved method, a quantity or calcium sulphite, as produced commercially by thepreferred meth od disclosed in the Haywood patents and applicationpreviously mentioned, was subjected to a series or different treatments in accordance *with the present invention. A sample of this material was treated with sulphuric acid in sufil- 'and brighter.

cient quantity to convert approximately 20% of the calcium sulphite into the sulphate. Another sample of the same original material was treated I assing treatment with substantially 100% pure sulphur dioxide in'suflicient amount to react the a original lime with an excess of 0.2'mol of sulphur dioxide. As far as possible the same conditions 1 of operation were maintained in the conduct of all ot these treatments." The residual, unconverted calcium sulphite remaining after each or these treatments wasthen subjected to various tests. So also a sample of the original; untreated calcium sulphite was subiectedto the same tests. These tests showed the vergassed and acid treated materials tube gr tly superior to the ordinary, untreated calcium te in many respects. The treated materials were much bulkier Hand-coated sheets of paper were produced, making use or the several treated and untreated Y with untreated material was considerably worse than the others. The coating was removed by even the least tacky of the graded waxes. The

treated materials, on the other hand, were bound much more firmly by the casein to the fibrous stock. A much more tacky wax was required to.

remove them and in removing the coatings with stock itself was removed.

-When excess sulphur dioxide is used to .con- 'vert a portion of the original, normal calcium sulphite into calcium bisulphite, it has been found sulphite into the bisulphite, as a result or the use of higher excesses of 80:, the yield of the final calciumsulphite will be lower. The table which will be shortly giveh shows the results obtained from the treatment of milk of lime at r 112 g./l. concentration with varying quantities of S02. In each case the milk oflime was whipped into. a fine spray in an atmosphere Of'SOs. One

sample was subjected to reaction with simply a molecular equivalent of S02, the next sample was subjected to treatment with a 12% excess of $02 over the molecular equivalent, the next sample to a 25% excess of 802, the next to a 50% excess and the last to a excess. The percentage of yield in each case is indicated in the second column or the table. The settling point, after 2% hours, oi 10 gms. of each of the products in 100 'cc. of distilled water, ai'ter being thoroughly shaken, is indicated in the third column; Handsheets of paper coated with each of the substances, utilizing 13% by weight of casein,'were formed from each of the samples and subjected to brightness, gloss, and Dennison wax tests. The brightness was determined by a General Electric brightnesetester, the coating applied to the hand-sheets for the purpose of thistest being equivalent to a 15 lb. coating on a ream of .500 sheets of paper 25 x 38 inches; the gloss was determined by a Bausch 8: Lomb glarimeter, and the Dennison wax'testwas conducted in the conventional way with the Dennison graded, waxes to-determine which wax would first cause the these other substances, a portion of the fibrous brightness, gloss and i when used as a coating coated surface to pick or pull. 1 The results of these tests were as follows:

It will be noted from the foregoing that at some point between the 12 and-25% excess of S02 a very remarkable increase in the bulk of the product resulted. A considerable improvement in brightness and gloss also resulted. Moreover, in the use of a relativelysmall quantity of casein, -i. e., 13 the products formed by the use of an excess of more than 12% of S02 were firm to a relation to tate.

2. A method of producing finely-divided calcium sulphite for filler and pigment purposes which comprises precipitating calcium sulphite from a lime bearing suspension by reaction of said suspension with sulphite ions, and subjecting the concentration of the said precipithe resulting precipitateat-a concentration of quite tacky wax, i. e., No. 6 or higher, equivalent to a very tacky ink. Even the 12% excess S02 very largely increased the firmness of the coating to the tacky waxes,

Other tests have shown that a considerable improvement results from the use of higher concentrations of the suspensions dealt with, particularly in the acidifying or over-gassing stage of the process. This improvement is most noticeable in the percentage of yield. For example,

an excess of 100% of SO: in the over-gassing process may be utilized in dealing with concentrations of the suspension in the neighborhood of 400 to 500 g./l. without dropping the yield below about 75%. At the same time the various characteristics of the product, such as its bulk,

ingredient, were improved to an even greater extent than in the use of the same percentage of excess of $02 at lower concentrations. Thisis apparently due to the fact. that a lower pH is maintained by the same excess at lower concentrations. In this connection it should also be observed that the cooling of the reacting materials to a point below 100 F., and

firmness to tacky waxes,

preferably to a point betwe nand as-' sists in retaining a grea r percentage of the excess S02 in solution and thus assists in maintaining a lower pH and enhances the results obtainable from the practice of the invention.

While the improved method and the characteristics of the resulting, improved product have been described in considerable detail, it will be understood that various modifications may be made in the steps and operating conditionsemployed and inthe specific characteristics of the product without departing from the general principles and scope of the invention. The terms and expressions employed herein have been used as terms of description and not of limitation.

Reference is hereby made to application Serial No. 383,148 by the present applicants.

What we claim is:

1. A method of producing finely-divided calcium sulphate for fillerand pigment purposes or the said precipitate is converted into a soluble salt, the extentzof the conversion being in inverse between 200 to 500 g./l. and at a temperature between 50 and F. totreatment with an acid until the pH of the resulting slurry is below approximately 31. 1 .;and approximately 10 to 40% of the said precipitate is converted into a soluble salt, the extent of the conversion being in inverse relation to the concentration of the said precipi tate.

3. A method of producing finely-divided cal cium sulphitefor filler and pigment purposes which comprises precipitating calcium sulphite from a lime bearing suspension by reaction of said suspension with sulphite ions, and subjecting the resulting precipitate at a concentration of between 200 to 500 g./l. to treatment with a gaseous medium which is more than 50 percent sulphur dioxide until the pH of the resulting slurry is below approximately 3.1 and approximately 10 to 40% of the said precipitate is converted into a soluble salt, the extent of the conversion being in inverse relation to the concentration of the said precipitate.

4. A method of producin finely-divided calrelation to the concentration of the said precipitate.

5. A method of producing finely divided calcium sulphite suitable for filler and pigment purposes, which comprises precipitating calcium sulphite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension with sulphit ions, treating the resulting precipitate at a concentration or between 200 and 500 g./l. with sulphuric acid until approximately 710 to 40% of said precipitate is converted into sulphate, and separating the residual precipitate from the acidified liquor.

6. A method of producing finely divided calcium sulphite suitable for filler and pigment'purposes, which comprises precipitating calcium slil' phite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension with a. gaseous medium which is between 10 and 20%sulphur dioxide, and" treating the resulting precipitate at a concentration of at least 200 g./l. with oxide until the pH below 4.0 and between 10 and 40% of the precipitate is converted to calcium bisulphite.

7. A method of producing finely divided calcium sulphite suitable for filler and pigment pursubstantially pure sulphur diposes, which comprises precipitating calcium sulphite from a calcium'bearmg suspension of an ionizable'calcium compound by reaction of said suspension with a gaseous medium containing from 7% to 20% sulphur dioxide, and subsequently subjecting the resuIting precipitate at a concentration of at least 200 g./l. to the action of the suspension is depressed caravan of a gaseous medium which is more than 50% suspension with sulphite ions, treating the resultsulphur dioxide, until the pH is depressed to below 4.0 and approximately 10% to 40% ot the precipitate has been dissolved.

' '8. A method of producing finely-divided calcium sulphitefor filler and pigment purposes ing precipitate with gaseous sulphur dioxide, converting approximately to 40% of the PM!!! tate with calcium bisuiphite, said treatment being conducted at a concentration of between 400 and 500 g./l. at a temperature between 50 1". and

which comprises precipitating calcium, sulphite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension.

with sulphite ions, subjecting the resulting precipitate at a concentration of between 200 to 500 g'./l. to treatment with an acid until the pH 0! the resulting slurry is below approximately 4 and approximately 10 to 40% of the said precipitate is converted into a soluble salt, and separating the residual precipitate. from the acidified" liquor.

9. A method of producing finely-divided calcium sulphite for illler'and pigment purposes which comprises precipitating calcium sulphite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension with sulflte ions, subjecting the resulting precipitate at a concentration of between 200 to 500 8J and at a temperature between 50 and 100 F. to

treatment with an acid until the pH of the resulting slurry is below approximately 4 and approxi-v mately 10 to 40% ofthe said precipitate is converted into a soluble salt, and separating the residual precipitate from the acidified liquor.

10. A method ofproducing finely-divided calcium sulphite for filler and pigment purposes which comprises precipitating calcium sulphite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension with sulphite ions, subjecting theresulting precipitate at a concentration of between 200 to 500 g./l. to treatment with a gaseous medium which is more than 50 per cent sulphur dioxide until the DH of the resulting slurry is below approximately 4 and approximately 10 to 40% of the said 100 F., the pH at the end Point being below 4.0,

. and separating the residual precipitate from the acidic liquor. J

14. A method of improving a finely-divided calcium sulphite to render it more suitable for filler and pigment purposes which comprises subjecting the calcium sulphite at a concentration 01' approximately '400to500 g./l. to treatment with an acid agent until approximately 10 to 40% of the sulphite is dissolved and the DH is below 4.0 and separating theresidual precipitate from the acidifled liquor.

15. A method of improving a finally-divided calcium sulphite to render it more suitable for filler and pigment purposes, which comprises subjecting the calcium sulphite at a concentration or between 400 and 500 grams per liter to reaction with sulphurous acid until approxiprecipitate is converted into a soluble salt, and

separating the residual precipitate from the acidifled liquor.

1 1. A method of producing finely-divided calcium sulphite for filler and pigment purposes which comprises precipitating calcium sulphite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension with sulphite ions, and subj cting the resulting precipitate at a concentration or between 200 to 500 g./l. to treatment" with alum until the pH "of the resulting slurry is below approximately 4 and approximately-10' to 40% of the said precipitate is converted into a soluble salt, and separatin the residual precipitate from the acidified liquor.

12. A method of producing a finely-divided calcium sulphite suitable for filler and pigment purposes which comprises precipitating calcium sulphite from a calcium bearing suspension 01' an ionizable calcium compound. by reaction-oi' said suspension with sulphite ions, subjecting the resulting precipitate at a concentration between 400 and 500 g./l. and at a temperature between F. and F. to treatment with an acid and converting approximately 10% to 40% 01' the mately 10 to 40% or the sulphite is converted to bisulphite and the pH is below 4.0.

16. A method of producing finely-divided calcium sulphite for filler and pigment purposes which comprises preparing a lime suspension containing at least grams of lime per liter of suspension, forming a spray of said suspension in an atmosphere containing sulphur dioxide, reacting said suspension with an excess or sulphur dioxide, saidexcess being greater than one-eighth of a moi for each moi of lime, and stopping said reaction after the DH of the suspension is below 4.0 and the lime after having been converted to calcium sulphite was partly converted to calcium bisulphite to the extent oi'approximately 10% to 40%; and separating the calcium sulphite iirom the acidic liquor.

111. A method of producing finely-divided calcium sulphite for filler and pigment purposes which comprises preparing a lime suspension containing at least 110 grams of lime per liter of suspension, forming a spray of said suspension in an atmosphere containing sulphur dioxide, reacting said suspension with. an excessfoi sulphur dioxide, said excess being greater than one eighth of 9. mol for each mol of lime, the concentration of sulphur dioxide in the atmosphere being increased irom the range oi 7% to 20% at the beto calcium bisulphite to the extent of approximately 10% to 40%, and separating the calcium sulphite from the acidic liquor.

18. A method of producing finely divided calcium sulphite suitable for filler and pigment pur-' precipitate into a soluble salt, the pH at the end oi the treatment being below-4.0, and separating the residual precipitate from the acidic liquor.

13. Amethod of producing finely-divided cal-- cium sulphite suitable for filler and pigment pur-' poses which comprises precipitating calcium. sulphite from a calcium bearing suspension of an ionizable calcium compound by reaction of said suspension with sulphite ions, treating the result-- ing precipitate at a concentration'of between 200 I, and 500 32/1. with sumcient alum to depress the pH below 4.0-and converting between 10 and40% poses which comprises precipitating calcium suiphite/ from a calcium bearing suspension of an ionizable calcium compound by reaction or said of the precipitate into calcium sulpho-aluminate and separating-the residual precipitate from the acidified liquor.

GERALD HAYWOOD. WRIGHT M. WELTON.

cgnirmtmm 9E co'RREcTI NQ a 'Ma 15 .19L;5.

{'01: filfi aboi re nhmb ered patqnt r eqixiring eorzfaet'i'op. as follows: Page 1, scond 'cqipinfi lifle 25, for th'ejatqnt umbei- 2,210,268

' -c01 1 n, line 57 for "net" z ead --wet page tii-st col umxig" lines 66 a nd 67, claim '1, {pr "sulphate" read '-'$u1p12 l1t6- l page j, second column, line 15,; c-Iaifil 11;}, r615 "aci mam-acid c ling-'13, dam 15 for finally [r ad ---,I:1nely--; and that the Qs aid Letters lratent' ghoum be mad with this I (Sean Figs}; Assistant flgnmiissio ner of Patents. 

